As a conventional motor, there is a known structure in which a magnet is fixed to an inner circumference surface or an outer circumference surface of a cylindrical yoke, and a stator is arranged to face the yoke so that the yoke and the stator rotate relatively to each other by a magnetic field that is generated between the magnet and the stator.
As this type of motor for example, there is an outer rotor type motor as shown in FIG. 13 that is a cross section in the axial direction, in which a magnet M is fixed or secured to the inner circumference surface of a yoke Y, and a stator (not shown) is arranged at the inner side of the magnet M in the radial direction with a gap. In addition, there is another motor shown in FIG. 14 for example, which is a cross section in the axial direction. This motor is an inner rotor type motor having a magnet M that is fixed or secured to an outer circumference surface of a yoke Y and a stator (not shown) that is arranged at the outer side of the magnet M in the radial direction with a gap. When fixing or securing the magnet M to the yoke Y, a position of the magnet M in the axial direction is fixed to that the lower end surface of the magnet M in the axial direction is one the same surface as the lower end surface of the yoke in the axial direction, or that the lower end surface of the magnet M in the axial direction protrude downward from the lower end surface of the yoke in the axial direction.
For example, the following methods are indicated as the manufacture method of this yoke. This manufacturing method includes forming a shaft hole at the center of a sheet iron, and pressing a part of the sheet iron so as to draw and form a cylindrical yoke. In this case, after the drawing process, the cylindrical molded member formed on the sheet iron is cut off from the sheet iron. This cutting can be performed by a method of exerting a shearing force in the axial direction along an imaginary line G or a method of exerting a shearing force in the radial direction along an imaginary line J as shown in FIG. 15 that is the cross section in the axial direction.
In the case where the shearing force is applied in the axial direction, a burr Sb that protrudes in the axial direction is formed at the lower end portion in the axial direction of the molded member S as shown in FIG. 16. As a result, it is difficult to make the dimension of the molded member S in the axial direction at a fine accuracy. In addition, in the case where the shearing force is applied in the radial direction, if the shearing force is applied outward the radial direction as shown in FIG. 17 for example, a burr Sb that protrudes outward in the radial direction is formed on the outer circumference surface of the molded member S. As a result, an annular member such as a magnet cannot be fixed to the outer circumference surface of the molded member S, for example. Therefore, a process is necessary for removing the burr after cutting in either case where the shearing force is applied in the axial direction or in the radial direction.
However, the additional process for removing a burr in the conventional method as described above may increase the number of manufacturing steps and a production cost.
In addition, the method of cutting the outward end portion in the radial direction of the molded member in the axial direction is difficult to apply to a small and low profile type of motor that uses the molded member as a yoke. A magnet is attached to the inner circumference surface or the outer circumference surface of the yoke of the small and low profile type motor, and position detection means such as a Hall element or the like attached to a circuit board or the like is provided immediately below the magnet. The Hall element is generally provided immediately below the magnet and a position that is closer to the magnet so as to detect a change of magnetic flux more precisely, which is generated by rotation of the magnet in the circumferential direction. However, if the yoke is cut in the axial direction, a burr is formed in the axial direction, and a distortion and the like is formed at the opening end portion of the yoke due to the shearing force for cutting. As a result, a dimension of the yoke in the axial direction cannot be finished precisely. If the dimension of the yoke in the axial direction cannot be finished precisely, a distance between the magnet and the position detection means such as a Hall element disposed adjacent thereto becomes uneven along the circumferential direction. As a result, the position detection means such as a Hall element cannot detect a magnetic flux of the magnet, and reliability of the motor is deteriorated.